1. Field of the Invention
The present invention relates to power supply apparatuses for supplying electric power to a capacitive load. The invention also relates to a method for operating such power supply apparatuses having a capacitive load such as an ozone generating device coupled to the power supply apparatus. Furthermore the invention relates to a high voltage transformer suitable for use in such power supply apparatuses.
2. Description of the Related Art
An example of capacitive loads is an ozone generating device coupled to a power supply apparatus generating an AC voltage to be supplied to the ozone generating device. Such power supply apparatuses have an inductive output impedance, and when the ozone generating device is connected to the output of the power supply apparatus, the inductive output impedance of the power supply apparatus and the capacitive impedance of the ozone generating device form a resonance circuit having a resonance frequency. Such ozone generating devices are driven at frequencies and voltages that are sufficiently high to produce a corona discharge in the ozone generating device. Air containing oxygen (O2) such as atmospheric air or pure oxygen is supplied to the ozone generating device, the corona converts oxygen molecules (O2) in the ozone generating device to ozone (O3), and air with an enhanced content of ozone in comparison to the air supplied to the ozone generating device is supplied from the ozone generating device. The amount of ozone produced by the ozone generating device increases with the voltage supplied to it, and for minimizing losses in the supply apparatus driving the ozone generating device the power supply apparatus should be operated at or near the resonance frequency. In practice, however, for several reasons the resonance frequency may not be constant and may vary over time and as a function of operating parameters including temperature and pressure in the supplied air/oxygen; exchanging the ozone generating device or parts thereof, e.g. for service or maintenance, may change the resonance frequency due to differences or tolerances in capacitance; and the resonance frequency may also change with the voltage at which the ozone generating device is operated since the corona is a non-linear phenomenon. It would therefore be advantageous to have a power supply apparatus which operates at the actual resonance frequency of the resonance circuit and which adapts its frequency of operation to the actual resonance frequency of the resonance circuit.
Ozone generating devices may be operated at voltage levels in the range of several kV, at frequencies of several kHz and at power levels of several kW. The power supply apparatus may have a high voltage transformer with a high voltage second coil as its output. When designing high voltage and high frequency transformers special considerations should be paid to the design of in particular the high voltage coil to avoid arcing between windings of the high voltage coil and between the windings and other objects near the coils. Arcing itself may damage the high voltage coil and other components, but arcing will create ozone which may have undesired effects on the equipment and the environment. It would therefore be advantageous to have a high voltage transformer with a high voltage coil where arcing between windings of the high voltage coil is reduced or even avoided.
On a commercial and industrial scale ozone is produced from oxygen, O2, in a gas containing oxygen. The oxygen-containing gas can be atmospheric air or oxygen-enriched gas. Methods exist for extracting oxygen from atmospheric air to produce oxygen-enriched gas. Ozone can be produced from oxygen mainly by two methods, one comprising irradiating the oxygen with ultra violet light, the other comprising a corona discharge device. Providing oxygen-enriched gas and producing ozone from oxygen are processes that consume energy and the consumptions of energy and other resources of the two processes are comparable.
In some applications where ozone is used a predetermined yield of ozone is needed or prescribed, or the required yield of ozone may change. A simple and straightforward way of adjusting the yield is to adjust only the electric power of the ozone-generating apparatus and leaving the flow or supply of oxygen-containing gas constant, or vice versa. This is not optimized for minimizing the consumption of resources comprising oxygen-containing gas and power supplied from the power supply apparatus, and the desired yield may possibly not result or may even be impossible to obtain.